1. Field of the Invention
The invention relates to a screw tap and a method for the production of a screw tap.
2. Background and Relevant Art
From the “Handbuch der Gewindetechnik and Frästechnik” (“Manual of Threading Practice and Milling Practice”, Publisher: EMUGE-FRANKEN, Publishing House: Publicis Corporate Publishing, Year of Publication: 2004 (ISBN 3-89578-232-7), hereinafter referred to simply as the “EMUGE Manual”, in Chapter 8, Pages 181 to 298, various embodiments of screw taps and tapping methods are disclosed.
Screw taps are tools for the cutting production of threads, which at one end can be fastened by a shank in a tool holder or chuck and at the other end have a working region, with thread cutters or thread-cutting teeth for cutting the thread into the workpiece. The thread-cutting teeth are arranged at a distance apart along a spiral or screw line, the pitch of which corresponds to the thread to be produced. In the cross section perpendicular to the cutting direction or to the screw line, the thread-cutting teeth are matched to the thread profile to be produced and therefore have, on the radially outermost tooth tip, outer cutters or tip cutters for the cutting of the thread bottom and, at the side, generally flank cutters for the cutting of thread flanks.
A screw tap generally has a lead region, in which the maximum radial distance of the tip cutters of the thread-cutting teeth increases from the end of the screw tap axially rearward in a linear or stepped manner, and, in addition, a guide region, which axially adjoins the lead region and in which the radial distance of the tip cutters of the thread-cutting teeth initially remains constant and then normally decreases again slightly conically. For the lead region, with respect to its chamfer length, chamfer diameter and chamfer angle, different lead forms are known, the chamfer length being relevant with respect to the length of the threaded holes. According to DIN, there are lead forms A, B, C, D and E, which differ in terms of the number of turns in the lead, the lead region, and in terms of the entering angle. Lead form A has, for instance, six to eight turns in the lead region and an entering angle of about 5°, lead form B a number of from 3.5 to 5.5 turns in the lead region and an entering angle of 8°, and lead form C a number of turns from two to three and an entering angle of 15°.
In screw tapping, the screw tap is rotated about its longitudinal axis as the rotational axis and, at the same time, is moved into the workpiece with a, relative to the rotational axis, axial feed motion, the axial feed rate being dependent on the rotation speed and the pitch. With screw taps, internal threads are produced in pre-machined through-bores or even blind holes or bottom holes, the thread-cutting teeth being continuously in engagement with the workpiece surface (continuous cut). For chip removal, the screw taps generally have chip grooves between adjacent thread-cutting teeth, which chip grooves can run straight or axially to the rotational axis or even spirally in the sense of rotation of the screw tap or oppositely to the sense of rotation. A screw tap can cut only in one cutting direction (clockwise rotation or counterclockwise rotation) and thus produce either only right-hand threads or only left-hand threads. In the cutting operation or screw tapping, the screw tap, when turned into the bore of the workpiece, makes a starting cut up to the engagement of all lead thread-cutting teeth, and the screw tap is then slowed down up to the maximum penetration depth. Once the whole of the thread is cut into the workpiece, the screw tap is turned back out of the produced thread, by reversal of the direction of rotation and of the axial direction of feed, in a rearward motion or return run. In the return run, the cuttings of the follow-up cutter which remain in the bore are sheared off with the land of the screw tap. In the onward rearward motion, the chip root which remains following shearing-off of the chips is further squeezed back into a gap whose size is dependent on the clearance angle of the screw tap. Next, in a further rearward motion, under the effect of the sliding friction, the screw tap is turned wholly out of the workpiece.
As material for the screw tap, in most cases, at least as the cutting material in the cutting part or on the working region, high-speed steels, in particular HSS for normal load or HSS-E for higher load, are used, though PM steels can also be used.
In addition, hard metal screw taps are also known, hard metal being taken to mean sintered or cemented metal carbides, in particular tungsten carbide, where necessary alloyed or mixed with metals or other metal carbides, solid hard metal (SHM) being spoken of in respect of screw taps in which shank and working region consist of hard metal, and tip hard metal (THM) in respect of screw taps in which the cutter part consists of hard metal and the shank of tool steel. Soldered-in, screwed-in or clamped hard metal strips with thread-cutting teeth are also known.
Because of their greater material hardness and higher compressive strength, and their greater temperature stability, hard metal screw taps have advantages over high-speed steel screw taps, for instance, in theory, a higher rotation speed and longer service life. Hard metal screw taps are advantageously used to bore threads in grey cast iron (GCI) or aluminum. However, hard metal screw taps have a relatively short service life in steels, which is normally less than with comparable HSS or HSS-E screw taps. The shorter service lives with hard metal screw taps presumably stem from the fact that the thread cutters, because of the higher brittleness and lower elasticity, as well as lower breaking strength and toughness of hard metal relative to high-speed steel, break or partially tear off or are worn down prematurely.
In order to increase the service lives and reduce the sliding resistance and build-up tendency, screw taps made both of high-speed steel and hard metal are often additionally surface-treated, from simple nitration up to a modern hard material coating, for instance hard chrome plating, coating with chromium nitride, titanium nitride, titanium carbonitride or titanium aluminum nitride.
In U.S. Pat. No. 7,147,939 B2, in order to increase the service life, a hard metal screw tap having a tungsten carbide core is alloyed with cobalt within a range from 14 to 16% by weight and with a wear-resistant coating, provided with a gradient, of metal nitride, carbide, carbon nitride, boride and/or oxide, the metal being aluminum, silicon or a transition metal from one of the periodic system groups IVa, Va and VIa, as well as with an outer coating containing molybdenum disulphide for reducing friction over the wear-resistant coating, is proposed. It is stated that the service life in 33 HRC AISI 4340 steel was higher than with an HSS screw tap conventionally coated with titanium nitride.
In U.S. Pat. No. 7,147,413 B2 and associated U.S. Pat. No. 7,207,867 B2, in order to increase the service life, a hard metal screw tap is proposed, comprising a cylindrical shank and a thread-producing region with a lead region containing a wear-resistant coating of metal nitride, carbide, carbon nitride, boride and/or oxide, the metal being aluminum, silicon or a transition metal of the periodic system groups IVa, Va and VIa and being coated with a further outer coating containing molybdenum disulphide. The cylindrical shank is held, during the grinding, with a hydraulic precision holder, so that the thread-producing region and the lead region are concentric to the cylindrical shank within a tolerance of 10 μm.
In the grinding of screw taps, those cutting edges of the thread-cutting teeth which point in the cutting direction are made as sharp as possible in order to enable a sharp and smooth cut of the thread, or, in other words, to enable roundings on the cutting edges to be kept as small as possible.
In the grinding of high-speed steel screw taps, burrs are now, however, generally formed on the cutting edges, which burrs lead to poor threads at the start of the period of use of the screw tap. It is therefore known, prior to first use of the screw tap, to remove the burrs on the cutters by a deburring operation with brushes or by jet-grinding or jet-machining with abrasive material or with a high pressure water jet. The cutting edges of the high-speed steel screw tap are thereby slightly rounded, however.
In the case of hard metal screw taps, no burr is formed in the grinding of the thread-cutting edges, since hard metal chips differently and has different deformation characteristics than high-speed steel when ground. Nor, therefore, with a hard metal screw tap, is there need for a deburring operation.
If hard material coatings are additionally applied to the material of the screw tap, the cutting edges are likewise rounded off slightly.
A wear-induced distinct rounding is then obtained during use of the screw tap, for which reason screw taps are then also frequently reground in order to recreate sharp cutting edges.
According to the prior art, in screw taps the aforementioned technologically conditioned, yet intrinsically undesirable roundings on the cutting edges are kept as small as possible, typically below a radius of curvature in the order of magnitude of 1 μm to maximally 10 μm. Larger radii of curvature and thus smaller curvatures are regarded by professionals as wear which would render the screw tap unusable.